FIG. 1 schematically shows a conventional rail-to-rail input stage 100, which has two stages: an NPN stage (Q1, Q2) that turns on at a first time (A stage), and a PNP stage (Q3-Q8) that turns on when the NPN stage turns off (B stage). As known by those skilled in the art, the input stage should have a substantially constant small signal transconductance gain (gm) to ensure that an appropriate signal is forwarded to a subsequent stage. To achieve a constant transconductance gain, the two stages are switched so that as the NPN stage begins to shut down, the PNP stage begins to become active, and vice versa. For example, the NPN A stage may be active when the common mode input voltage approaches the positive rail voltage. Conversely, the PNP B stage may be active when the common mode input voltage approaches the negative rail voltage. To smoothly effectuate this switch at higher total supply voltages (i.e., greater than about three volts), a switch transistor Q15 (control circuitry 105) steers a tail current between the two stages as a function of the input common mode voltage. This tail current is switched using a current mirror formed by Q19 and Q18.
Problems arise, however, when making the transition between stages at lower total supply voltages. At lower total supply voltages, the PNP stage begins to turn off much earlier than the NPN stage turns on, i.e., the PNP stage turns off, for example as shown in FIG. 1 at VCC-3 Vbe (approximately 0.7V) while the NPN stage of FIG. 1 still has to wait for a voltage of about 2 Vbe (1.4 V) to become active. This phenomenon undesirably causes a “dead zone” region 200 at about the middle of the input range, which is shown by example in FIG. 2. In the dead zone region 200 for voltage changes there is no current output and therefore the gain is zero. Accordingly, if the input stage is used as an input stage for a comparator or an operational amplifier, for example, the second stage of said comparator or amplifier will not function properly because it is not receiving a proper input signal